This invention relates to a novel method for photoexposing a coated sheet in a vacuum printing frame prior to etching. The novel method may be used, for example, in preparing slit or apertured masks for use in color television tubes and display picture tubes. The invention includes a novel printing plate which is especially useful in the novel method.
The preparation of apertured masks by photoexposure and etching has been described previously; for example, in U.S. Pat. Nos. 3,199,430 to S. A. Brown, 3,313,225 to N. B. Mears, 3,751,250 to J. J. Moscony et al. and 4,061,529 to A. Goldman et al. In a typical process, a thin sheet of metal, such as cold-rolled steel or a nickel-iron alloy, is coated on both major surfaces with a light-sensitive resist or enamel. Then, the coated sheet is positioned between two glass photographic printing plates, each plate carrying an opaque photographic master pattern on its inward-facing surface, each master pattern being accurately positioned with respect to the other. Each plate is usually comprised of a central area bearing the photographic master pattern and a peripheral area which is clear. The glass plates are mounted in spaced relation from one another in an equipment referred to as a vacuum printing frame. When the coated sheet is in the desired position, the printing frame is evacuated whereby the master patterns on the glass plates are pressed firmly against the coatings on the sheet. Then, the coatings are exposed to actinic radiation which passes through the glass plates for a time interval until the coatings are suitably exposed. The frame is then devacuated; that is, brought back to atmospheric pressure. The glass plates are separated and the metal sheet with the exposed coatings thereon is removed from the frame. The exposed coatings may now be developed by removing the more-soluble portions thereof. Then, the sheet may be selectively etched, and finally the less-soluble portions of the coatings are removed from the sheet.
One of the problems encountered previously is the peripheral areas of the glass plates flex sufficiently to contact each other and increase the evacuation and devacuation times. In some prior structures the transparent portion of the emulsion supporting the master patterns extended into the peripheral areas of the glass plates. Grooves were sometimes randomly cut in the emulsions on the peripheral areas of the plates to speed the evacuation and devacuation process; however neither the locations nor the dimensions of the grooves were sufficiently controlled to substantially improve the evacuation and devacuation times. U.S. Pat. No. 3,751,250, referenced above, attempted to shorten the evacuation and devacuation times by removing at least a portion of the surface of the peripheral areas of the glass plates to a depth of between 0.076 mm (0.003 inch) and 0.254 mm (0.010 inch). Even on glass plates where the entire peripheral areas were removed to the above-described depth, seal-off due to plate flexing sometimes occurred. Additionally, the removal of glass in the peripheral areas of the plates introduced stress into the glass which subsequently lead to cracking of the glass plates.
U.S. Pat. No. 4,588,676 issued on May 13, 1986 to J. J. Moscony et al. describes a method for exposing a coating of photoresist on a metal sheet using glass plates having a very thin, uniform thickness opaque master pattern formed on the major surfaces of the glass plates which contact the coated sheet. The use of thin master patterns of the type described in U.S. Pat. No. 4,588,676 increases the evacuation and devacuation problem because even less flexing of the glass plates is required to seal-off the plates. To shorten the times for evacuating and devacuating the printing frame, transparent islands of rubbery material are provided on the patterned surfaces of the printing plates. The islands are allowed to overlap slightly into the peripheral area of the plate immediately adjacent to the master pattern. The islands are typically about 2 micrometers thick and spaced about 0.3 mm to 0.8 mm apart center-to-center. Experience has shown that the resulting gap is insufficient to consistently provide rapid evacuation and devacuation times. It is impractical to increase the height of the islands sufficiently or to otherwise increase the spacing between the plates and the coated sheet since such expedients, while decreasing the evacuation and devacuation times, will also increase the longitudinal spacing between the opaque elements of the master pattern and the coated metal sheet, thereby reducing the resolution or sharpness of the projected master pattern onto the coated sheet.